Heretofore, rigid thermoplastic resin foams have been widely used as heat-insulating materials for ordinary houses and other buildings because of such excellent properties as a low head conductivity, a low water-absorbing property, a light weight and a good processability. However, since these foams are poor in the flexibility and have a large compression set, for example, when these foams are filled between pillars in buildings, it is necessary to cut the foams into a size corresponding to the distance between the pillars and fix the cut foams by special metal fittings. Practically, deviations of the distance between the pillars are not compensated, the operation efficiency is very low, and an air-tight, heat-insulating structure cannot be obtained. Furthermore, since they are rigid, a large force is necessary for filling the foams air-tightly, and application of a large force results in breakage of cells. Moreover, it happens that even the foams per se are broken, and an air-tight, heat-insulating structure cannot be obtained.
Recently, the heights of houses and buildings are increased, and especially in such buildings as multi-storied apartment houses, a high sound-insulating property is required between two adjacent stories. In order to solve this problem, there has recently been adopted a floating floor construction method in which an inorganic fiber board is arranged on a floor base and a concrete mortar is deposited on the inorganic fiber board through a waterproof layer. According to this method, the flexibility of the inorganic fiber board is utilized and solid-propagative sounds between upper and lower stories are reduced, and the sound-insulating property can be enhanced. When the conventional rigid thermoplastic resin foams are used in this method, since they are rigid, no sufficient sound-insulating property can be obtained.
As means for eliminating the foregoing disadvantages, there has recently been proposed a foam obtained by mechanically softening a polystyrene foam having a reduced density. In this foam, however, if the density is lower than 20 Kg/m.sup.3, the resin wall defining cells is broken by the mechanical softening treatment, and a so-called communicating phenomenon takes place. Furthermore, even if breakage of cells in a relatively thick surface layer of the foam is controlled and an apparent closed cell ratio judged from the water absorption determined by an ordinary measuring method is maintained, cells which are present in the interior of the foam and composed of a wall thinner than the wall of the surface layer are broken, and the foam is inferior in the heat-insulating property and the maintenance of the heat-insulating property for a long time and the temperature dependency of the heat-insulating property increases. Furthermore, if the mechanical softening treatment is carried out, the foam as a whole is softened, and microscopically, the cells of the foam include softened portions and unsoftened portions and the physical properties of the foam are not uniform in the thickness direction or plane direction of the foam.
Plates or molded articles obtained by blowing foamable particles containing a foaming agent several times separately and finally effecting blowing in a mold are used as heat-insulating, shock-absorbing or sound-insulating materials. However, in a foam having a low density (a high blowing ratio) which is obtained by increasing the frequency of the operation of blowing the particles, since the size of the particles per se is increased, air-tight filling is impossible when a molded article is formed, and the fusion bonding among the particles is weak in the obtained foam. In order to obtain a strong fusion bonding, it is necessary to perform compressive filling and compressive molding when a molded article is formed, and the obtained foam is free of wrinkles referred to in the present invention and is poor in the flexibility and sound-insulating property. Accordingly, in the above-mentioned method, reduction of the density is restricted and the density of a foam that can be put into practical use is 17 Kg/m.sup.3 or higher.
However, it has long been desired in the market to develop a rigid thermoplastic resin foam having a low density, a high flexibility and a good heat-insulating property, where the heat-insulating property is maintained for a long time, the temperature dependency of the heat-insulating property is small, the sound-insulating property is good and the compression set is reduced.